1. Field of the Invention
This invention relates to actinic radiation-sensitive elements for colored image reproduction and more particularly to elements wherein positive images are formed for the purposes of proofing the color separations of original, multi-colored, renditions of graphic art.
2. Description of the Prior Art
In the photomechanical process of reproducing the tone values and color shades represented by a piece of original graphic art, it is necessary to prepare a series of color separations by photographic means so that the particular colors to be printed are first recorded on a set of separation negatives or postives by means of appropriate filtering and masking techniques.
By following known procedures, four photographic black and white separation negative or positives are made representing the three process colors, magenta, cyan and yellow as well as the fourth representing the black contribution. These separations are then used to make four printing plates. These plates are then mounted on an offset printing press to recreate a rendition of the original piece of art.
In some instances, the printing press is not faithful in its reproduction of the copy carried by the printing plates because of inherent limitations in the printing process. In other instances, the buyer of the printed copy may desire changes in tonal values so that the printed matter as represented by the output of the press is more satisfying to his customers. For these reasons and others, a method of previewing the results before actually printing the subject matter has been devised whereby the changes and adjustments may be made to the separation transparencies prior to the manufacture of the printing plates.
One method of previewing the color separation of an original colored pieces of art work is disclosed in U.S. Pat. No. 3,136,637. This patent discloses the use of a multiple layered negative acting proofing system. In this system each of the colored sheets which have been exposed with the appropriate separation negative are developed and then superimposed one on top of the other in exact register. These transparent sheets containing the colored images in exact detail can then be examined individually or in total to recognize any deficiencies compared to the original piece of graphic art prior to the manufacture of the printing plates.
In U.S. Pat. No. 3,671,236, the same objective as above is attained by superimposing on a single substrate the several colored layers so that the composite structure more closely resembles the final printed specimen. This process is again a negative acting system wherein a litographic negative is the means whereby the information is transferred photomechanically to the colored proofing sheets.
Still another proofing system is disclosed in U.S. Pat. No. 2,993,788 wherein a lithographic negative is used to expose a positive acting diazo oxide sensitized substrate. By exposing, developing and the application of a dye of predetermined color using the non-exposed portions of the diazo oxide as a photomask, a single sheet, negative acting proofing composition is attained.
In an examination of the referenced methods of generating color proofs from either positive or negative separations, a common problem is always cited. Namely, that of preserving the background or non-imaged portions of the proof from unwanted staining or discoloration (i.e. toning), which degrades the overall value of the composite proof.
The present invention relates to a presensitized sheet material and an associated proofing method which substantially eliminates such discoloration in the proofing sheet.